Pump with plunger having tribological coating

ABSTRACT

A pump is disclosed. The pump may include at least one pumping mechanism. The at least one pumping mechanism may include a barrel formed of a substrate having a bore and a plunger formed of a substrate and slidably disposed within the bore in the barrel. The pump may further include a coating disposed on the plunger. The coating may include a main layer containing a tribological material and a sacrificial break-in layer disposed on the main layer, the break-in layer containing a tribological material.

TECHNICAL FIELD

The present disclosure relates generally to a pump and, moreparticularly, to a pump with a plunger having a tribological coating.

BACKGROUND

Gaseous fuel powered engines are common in many applications. Forexample, the engine of a locomotive can be powered by natural gas (oranother gaseous fuel) alone, or by a mixture of natural gas and dieselfuel. Natural gas may be more abundant and, therefore, less expensivethan diesel fuel. In addition, natural gas may burn cleaner in someapplications.

Natural gas, when used in a mobile application, may be stored in aliquid state onboard the associated machine. This may require thenatural gas to be stored at cold temperatures, typically about −100 to−162° C. The liquefied natural gas (LNG) may then be drawn from the tankby gravity and/or by a boost pump and directed to a high-pressure pump.The high-pressure pump further increases a pressure of the fuel anddirects the fuel to the machine's engine. In some applications, theliquid fuel is gasified prior to injection into the engine and/or mixedwith diesel fuel (or another fuel) before combustion.

One problem associated with conventional high-pressure pumps involveslubricating the moving parts of the pump. Generally tight tolerancesbetween moving parts, such as between plungers that reciprocate withinbarrels of pumping mechanisms, may create friction between the movingparts, thereby requiring more energy to drive the pump. Further, thefriction may cause scuffing, wearing, and/or sticking of the plungers,barrels, or seals between the plungers and barrels, which can reduce thelifespan of the pump. Whereas diesel fuel may be more naturallylubricious and may lubricate plungers, barrels, and seals duringoperation of the pump, gaseous fuels generally have lower lubricity andmay not provide sufficient lubrication.

One attempt to reduce friction within a fuel pump is disclosed in U.S.Pat. No. 7,134,851 (the '851 patent) that issued to Chenoweth on Nov.14, 2006. In particular, the '851 patent discloses a reciprocating pumphaving a pump body and a plunger housing disposed in the pump body. Theplunger housing defines a bore, and a plunger slides within the bore todraw fuel through an inlet of the pump and force the fuel through anoutlet. A relatively small clearance is maintained between the plungerand the bore of the plunger housing to prevent leakage past the plunger.The plunger and/or plunger housing are formed of a ceramic material toreduce wear of the plunger and the cylinder.

While the pump of the '851 patent may reduce some fuel leakage andwearing of the plunger and/or plunger housing, it may not be suitablefor primping low-temperature cryogenic fluids, such as LNG.Particularly, ceramic plungers and plunger housings may not besufficiently lubricious at cryogenic temperatures to prevent wearing orsticking. Further, a difference in thermal expansion between ceramicplungers and plunger housings over a range of working temperatures maycause fuel leakage or wearing, thereby reducing the overall efficiencyof the pump.

The disclosed pump is directed to overcoming one or more of the problemsset forth above and/or other problems of the prior art.

SUMMARY

In one aspect, the present disclosure is directed to a pump. The pumpmay include at least one pumping mechanism. The at least one pumpingmechanism may include a barrel formed of a substrate having a bore and aplunger formed of a substrate and slidably disposed within the bore inthe barrel. The pump may further include a coating disposed on theplunger. The coating may include a main layer containing a tribologicalmaterial and a sacrificial break-in layer disposed on the main layer,the break-in layer containing a tribological material.

In another aspect, the present disclosure is directed to a method offorming a pump. The method may include providing a plunger and applyinga coating to the plunger. Applying the coating may include applying amain layer of the coating to the plunger, the main layer containing atribological material. Applying the coating may also include applying asacrificial break-in layer of the coating to the main layer, thebreak-in layer containing a tribological coating. The method may furtherinclude slidably disposing the plunger within a bore, the bore beinglocated in a barrel.

In yet another aspect, the present disclosure is directed to a pump. Thepump may include at least one pumping mechanism configured to be fondlyconnected to a source of cryogenic fluid. The at least one pumpingmechanism may include a barrel formed of stainless steel. The barrel mayinclude a bore having a nickel-plated surface. The at least one pumpingmechanism may further include a plunger formed of stainless steel, theplunger being slidably disposed within the bore in the barrel. The atleast one pumping mechanism may be configured to pressurize thecryogenic fluid between the plunger and the barrel. The pump may furtherinclude a coating disposed on the plunger. The coating may include asupport layer containing a diamond-like carbon (DLC) material. Thecoating may also include a main layer disposed on the support layer, themain layer containing amorphous diamond-like carbon (ADLC). The coatingmay further include a sacrificial break-in layer disposed on the mainlayer, the break-in layer containing a DLC material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary disclosed pump;

FIG. 2 is a cross sectional view of an exemplary disclosed pumpingmechanism of the pump of FIG. 1;

FIG. 3 is a sectioned perspective view of an exemplary disclosedcylinder of the pumping mechanism of FIG. 2;

FIG. 4 is a sectioned perspective view of an exemplary disclosed plungerof the pumping mechanism of FIG. 2; and

FIG. 5 is a flow chart showing an exemplary method of forming the pumpof FIG. 1.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments that areillustrated in the accompanying drawings. Wherever possible, the samereference numbers will be used throughout the drawings to refer to thesame or like parts.

FIG. 1 illustrates a pump 10 that may be used to supply a pressurizedfluid. For example, pump 10 may be a pump used to provide pressurizedfuel, such as a cryogenic fluid (e.g., liquefied natural gas (LNG),helium, hydrogen, nitrogen, or oxygen) to a fuel consumer, such as agaseous fuel-powered engine. It is contemplated, however, that pump 10may supply other gaseous fuel consumers.

Pump 10 may be mechanically driven by an external source of power (e.g.,an engine or a motor) at an input end 12 to generate a high-pressurefluid discharge at an output end 14. In this example, input end 12 andoutput end 14 may be aligned along a common axis 16, and connectedend-to-end. For example, pump 10 may be an axial plunger type pump.Other configurations may be possible.

Input end 12 may include a driveshaft 18 rotatably supported within ahousing (not shown), and connected at an internal end to a load plate20. Load plate 20 may be oriented at an oblique angle relative to axis16, such that an input rotation of driveshaft 18 may be converted into acorresponding undulating motion of load plate 20. A plurality of tappets(not shown) may slide along a lower face of load plate 20, and a pushrod 22 may be associated with each tappet. In this way, the undulatingmotion of load plate 20 may be transferred through the tappets to pushrods 22 and used to pressurize the fluid passing through pump 10. Aresilient member (not shown), for example a coil spring, may beassociated with each push rod 22 and configured to bias the associatedtappet into engagement with load plate 20. Each push rod 22 may be asingle-piece component or, alternatively, comprised of multiple pieces,as desired. Many different shaft/load plate configurations may bepossible, and the oblique angle of load plate 20 may be fixed orvariable, as desired. Other configurations of input end 12 may bepossible.

Output end 14 may be in fluid communication with a cryogenic firedsource via an inlet 24 and an outlet 26. For example, LNG may besupplied to output end 14 from an associated storage tank storing LNG attemperatures of, e.g., about −100 to −162° C. In some embodiments, LNGmay be supplied to output end 14 at less than about −140° C. or lessthan about −120° C. This continuous supply of cold fluid to output end14 may cause output end 14 to be significantly cooler than input end 12.Cryogenic fluid may be directed through inlet 24 to a reservoir 28 inoutput end 14.

Output end 14 may include one or more pumping mechanisms 30 fluidlyconnected to reservoir 28 to draw cryogenic fluid from reservoir 28. Inthe exemplary embodiment, output end 14 has five pumping mechanisms 30,but it is understood that there may be more or fewer than five pumpingmechanisms 30. Pumping mechanisms 30 may be mounted to a surface 32disposed in output end 14 and may extend into reservoir 28. Push rods 22may extend through surface 32 and extend into each pumping mechanism 30.

As shown in FIG. 2, each pumping mechanism 30 may include a barrelassembly 34 including a base or proximal end 36 and a distal end 38opposite proximal end 36. The terms “proximal” and “distal” are usedherein to refer to the relative positions of the components of exemplarybarrel assembly 34. When used herein, “proximal” refers to a positionrelatively closer to the end of barrel assembly 34 connected to surface32. In contrast, “distal” refers to a position relatively further awayfrom the end of the barrel assembly 34 that is connected to surface 32.

Barrel assembly 34 may include a generally hollow barrel 40 formed atproximal end 36 and a head 42 formed at distal end 38. Barrel 40 mayinclude a barrel substrate 58 (FIG. 3) formed of a suitable materialthat can withstand changes from ambient temperatures to cryogenictemperatures and continued operation at such temperatures. A suitablematerial may resist cracking or other mechanical failures and may navelow thermal expansion properties. For example, barrel 40 may be formedof a type of stainless steel that is suitable for operation at such lowtemperatures. In particular, barrel 40 may be formed of 17-4 PH H1150Mstainless steel. However, it is understood that barrel 40 may be formedof another material, such as another type of steel, a different metal,or a ceramic material.

Head 42 may be attached to barrel 40 to close off barrel 40.Alternatively, barrel assembly 34, including barrel 40 and head 42, maybe formed integrally as a single component. Head 42 may include at leastone inlet 44 in fluid communication with reservoir 28 for drawingcryogenic fluid from reservoir 28 into barrel assembly 34. Head 42 mayalso include at least one outlet 46 in fluid communication with outlet26 and an outlet conduit for transferring pressurized cryogenic fluidfrom barrel assembly 34 to the fuel consumer.

A plunger bore 48 may extend through barrel 40 and may be configured toreceive a plunger 50 for sliding within plunger bore 48. A proximal endof plunger bore 48 may also align with push rod 22 such that push rod 22may contact and slide plunger 50 through plunger bore 48. For example,the undulation of load plate 20 may cause push rod 22 to move toward andpush against plunger 50. Alternatively, plunger 50 may be connected topush rod 22, and the undulation of load plate 20 may cause push rod 22to move plunger 50. During the ensuing plunger movement, high pressuremay be generated within pumping mechanism 30 by the volume contractinginside plunger bore 48.

Plunger 50 may slide between a Bottom-Dead-Center position (BDC) and aTop-Dead-Center (TDC) position within plunger bore 48. While plungers 50reciprocate between BDC and TDC, fuel may leak past plungers 50 into theproximal ends of the respective plunger bores 48 due to the pressuredifferential between the pressure acting on the distal end of plunger 50and the pressure acting on the proximal end of plunger 50. Plunger 50may be formed without an external seal (e.g., a piston seal, such as aplastic or non-metallic component disposed on the outer surface of theplunger 50), which may wear and limit the life of the pump 10.Alternatively, plungers 50 may include the external seal or other typeof seal. To reduce leakage past plungers 50, a small clearance may bemaintained between plunger bore 48 and plunger 50.

To further reduce leakage past plungers 50, plungers 50 may include aplunger substrate 60 (FIG. 4) formed of a suitable material that canwithstand changes from ambient temperatures to cryogenic temperaturesand continued operation at such temperatures. A suitable material mayresist cracking or other mechanical failures and may have low thermalexpansion properties. In particular, a suitable material may be one withthe same or similar thermal expansion properties as the material formingbarrel 40. In this way, expansion and contraction of the materialforming plunger 50 and barrel 40 may occur at similar rates astemperatures change in pump 10, thereby maintaining a desired clearancebetween plunger bores 48 and plungers 50 to reduce leakage. For example,plungers 50 may be formed of a type of stainless steel that is suitablefor operation at such low temperatures. In particular, plungers 50 maybe formed of 17-4 PH H1150M stainless steel. However, it is understoodthat plungers 50 may be formed of another suitable material, such asanother type of steel a different metal, or a ceramic material.

The surfaces of barrel 40, including plunger bores 48, and plungers 50may be coated with wear resistant tribological materials that may reducefriction between plunger bores 48 and plungers 50 as plungers 50reciprocate between BDC and TDC. Tribological materials may includematerials that are used to reduce friction between and wearing ofsurfaces in sliding contact. In particular, tribological materials mayinclude wear resistant coatings, plating, lubricants (e.g., drylubricants, wet lubricants, etc.), and other protective materials thatmay be applied to a surface to reduce friction and wearing. Reducing thefriction between plunger bores 48 and plungers 50 may reduce or preventscuffing and/or wearing of plunger bores 48 and plungers 50, therebyincreasing the lifespan of pump 10. Reducing the friction may alsoinhibit plungers 50 from heating up and expanding within plunger bores48, allowing a small clearance between plungers 50 and plunger bores 48to be maintained and preventing plungers 50 from sticking within plungerbores 48. In this way, the efficiency of pump 10 may be improved.

FIGS. 3 and 4 show barrel 40 coated with a barrel coating 52 and plunger50 coated with a plunger coating 54. Barrel coating 52 and plungercoating 54 may be different coatings or the same coating, if desired.Each coating may include respective layers, and each layer may containdifferent materials or the same materials, if desired. Any number oflayers may be disposed on barrel 40 and plunger 50 as desired.

Barrel coating 52 may include at least a first layer 56 disposed onbarrel substrate 58. First layer 56 of barrel coating may include a hardmaterial for resisting wear under bearing forces, scuffing, and frictionbetween plunger bore 48 and plunger 50. First layer 56 of barrel coating52 may also include a material having a coefficient of thermal expansionthat matches or is sufficiently similar to that of barrel substrate 58to reduce cracking and/or delamination during temperature changes andoperation of pump 10. For example, first layer 56 of barrel coating 52may include metal plating such as, for example, nickel plating or chromeplating. In this example, first layer 56 of barrel coating 52 mayinclude nickel plating applied by an electroless process. Due to itshardness and toughness at low temperatures, electroless nickel platingmay be applied as a thin layer such that the likelihood of crackingduring thermal expansion may be reduced. It is understood, however, thata different metal plating applied by the same or another process may beused, if desired.

First layer 56 of barrel coating 52 may also be impregnated with othermaterials to provide additional or improved tribological properties(e.g., lubricity, wear resistance, hardness, etc.) to first layer 56.Additional materials may include materials that cart be impregnated infirst layer 56 and maintain lubricity at cryogenic temperatures. Suchtribological materials may include, for example, polytetrafluroethylene(PTFE), materials containing molybdenum disulfide (MoS₂), such astitanium molybdenum sulfide (TiMoS₂), and/or another suitable materials.It is understood that other tribological materials may be included infirst layer 56 of barrel coating 52. It is further understood thatadditional layers of barrel coating 52 may be included, if desired.

Plunger coating 54 may include a plurality of layers disposed on plungersubstrate 60. Due to the low clearance maintained between plunger bore48 and plunger, the layers of plunger coating 54 may be thin layers, andthe overall thickness of plunger coating 54 may be small enough to avoidthe effects of relative thermal expansion and reduce the likelihood ofcoating bond failure as well as increasing or decreasing the desiredclearance between plunger bores 48 and plungers 50. In this example,plunger coating may have an overall thickness of about 4-6 microns suchas, for example, about 5 microns. It is understood, however, thatplunger coating 54 may be thicker or thinner, if desired.

Plunger coating 54 may include a support layer 62 disposed on plungersubstrate 60, a main layer 64 disposed on support layer 62, and asacrificial break-in layer 66 disposed on main layer 64. Layers 62-66may include tribological materials and may cooperate with each other andwith barrel coating 52 to reduce friction and wearing between plungerbore 48 and plunger 50 during operation of pump 10 at cryogenictemperatures.

Support layer 62 may be disposed on plunger substrate 60 to providesupport to main layer 64 by absorbing some of the stresses caused bybearing forces and thermal expansion. Support layer 62 may be hard andwear resistant, but may be slightly less hard, thinner, and moreflexible than main layer 64. For example, support layer 62 may contain adiamond-like carbon (DLC) material applied to plunger 50 using aphysical vapor deposition (PVD) process, such as unbalanced magnetronsputtering.

In particular, support layer 62 may contain tungsten DLC (W-DLC) and maybe about 0.5-1.5 microns thick. Support layer 62 may be thinner orthicker, if desired, and it is understood that increasing the thicknessof support layer 62 may increase the effects of thermal expansionexperienced by support layer 62. The ratio of carbon to tungsten inW-DLC may be selected to achieve a desired compromise between wearresistance and toughness in support layer 62. For example, a greatertungsten-to-carbon ratio may improve wear resistance, but may increasebrittleness of support layer 62. Other metal-doped DLC materials such astitanium DLC (Ti-DLC), cobalt DLC (Co-DLC), copper DLC (Cu-DLC),chromium DLC (Cr-DLC), etc., may alternatively or additionally be used,if desired. It is understood that other tribological materials, such aschromium carbide, and other application process may be usedalternatively or in addition to a DLC material, if desired.

A thin adhesive layer of chrome (not shown) may be applied betweensubstrate 60 and support layer 62. Applying a thin layer of chrome toplunger substrate 60 may prevent delamination of support layer 62 andmay reduce stresses caused by thermal expansion differences betweenplunger substrate 60 (e.g., containing steel) and support layer 62.

Main layer 64 may be disposed on support layer 62 and provide a mainwear surface for lubricating sliding movements of plunger 50 withinplunger bore 48. Main layer 64 may contain tribological materials thatmay be harder, more wear resistant, and/or more lubricious than supportlayer 62. For example, main layer 64 may contain DLC materials such asamorphous diamond-like carbon (ADLC), applied using a plasma-assistedchemical vapor deposition (CVD) process, it is understood that othertribological materials and other application processes may be used ifdesired.

ADLC is relatively highly lubricious, hard, and wear resistant atcryogenic temperatures. For example, ADLC may have a Vickers hardness of2000-2200 HV. ADLC may also be applied in thin layers using theplasma-assisted CVD process. In this example, main layer 64 may be about1.5-3.0 microns (e.g., about 2-2.5 microns) thick. Such a relativelythin layer of ADLC may reduce the effects of thermal expansion, therebyreducing leakage between plunger bore 48 and plunger 50, and obviatingthe need far seals between plunger bore 48 and plunger 50.

On a nano-scale, main layer 64 (e.g., containing ADLC and/or otherrelatively harder material) may be relatively abrasive to plunger bores48 until plunger coating 54 and barrel coating 52 are broken-in, e.g.,after plunger 50 slides between BDC and TDC within plunger bore 48 for aperiod of time. However, the break-in process may substantially wearbarrel coating 52 and/or plunger bore 48, thereby decreasing thelifespan of pump 10. Accordingly, break-in layer 66 may be disposed onmain layer 64 to reduce wearing of barrel coating 52 during the break-inprocess.

Break-in layer 66 may be a thin layer (e.g., about 1-2 microns)containing a DLC material. Further, break-in layer 66 may be configuredto wear away from main layer 64 and be deposited on plunger bore 48during the break-in process. That is, break-in layer 66 may be asacrificial layer. In some embodiments, applying a DLC coating on top ofbarrel coating 52 may not be practical due to constraints of the DLCapplication process and to the geometry of barrel 40. Thus, break-inlayer 66 may be applied to main layer 64 and configured to wear awayfrom main layer and adhere to plunger bore 48. Alternatively, break-inlayer 66 may be formed on top of barrel coating 52 rather than on mainlayer 64.

Break-in layer 66 may contain one or more of a DLC material, such asW-DLC, and other tribological materials. As discussed above, thetungsten-to-carbon ratio of W-DLC may be adjusted to make break-in layer66 harder or softer as desired. In this example, break-in layer 66 maybe softer than support layer 62 and may include a lowertungsten-to-carbon ratio. By lowering this ratio, break-in layer 66 maybe soft enough to be transferred from main layer 64 to plunger bore 48to reduce the wearing of plunger bore 48 by main layer 64.

It is understood that other tribological materials may be usedalternatively or in addition to a DLC material in break-in layer 66, ifdesired. For example, the PVD process may be used to apply othertribological materials, such as, for example, MoS₂ or pure carbon.Alternatively, different processes may be used to apply differenttribological materials, such as TiMoS₂. The addition of titanium to MoS₂may increase the wear resistance of MoS₂, which is a highly lubriciousmaterial. Further, tribological materials containing ceramic materialsmay alternatively be used. For example, in place of a material appliedusing the PVD process, ceramic materials may be applied using a thermalspray process. In particular, break-in layer 66 may contain a zirconiathermal spray. The zirconia thermal spray may be lubricious at cryogenictemperatures but may be thicker than materials applied using the PVDprocess. The zirconia thermal spray may be, for example, about 50-100microns thick.

INDUSTRIAL APPLICABILITY

The disclosed pump finds potential application in any fluid system wherefriction and wearing of internal parts is undesirable, especially at lowtemperatures. The disclosed pump finds particular applicability incryogenic applications, for example power system applications havingengines that burn LNG fuel. One skilled in the art will recognize,however, that the disclosed pump could be utilized in relation to otherfluid systems that may or may not be associated with a power system. Amethod of forming pump 10 will now be explained with reference to FIG.5.

A method 400 of forming pump 10 may begin by providing barrel 40 andplunger 50 (Step 402) that have been formed from barrel substrate 58 andplunger substrate 60, respectively. In this example, barrel substrate 58and plunger substrate 60 may include stainless steel, such as 17-4 PHH1150M stainless steel, in other embodiments, barrel substrate 58 andplunger substrate 60 may include other types of stainless steel orceramic materials, if desired. Barrel substrate 58 and plunger substrate60 may include the same or different materials, if desired.

Due to the geometry of barrel 40 and plunger bore 48, barrel 40 andplunger 50 may be coated in separate steps. For example, first layer 56of metal plating may first be applied to plunger bore 48 of barrel 40(Step 404). It is understood that although barrel 40 is described asbeing coated before plunger 50, steps 404-414 may be performed in adifferent order, and plunger 50 may alternatively be coated beforebarrel 40. Metal plating, such as nickel plating, may be applied tobarrel 40 and plunger bore 48 using an electroless process. Theelectroless process is suitable for applying thin coatings of metalplating, which may reduce the effects of thermal expansion differencesbetween plunger bore 48 and plunger 50, as compared to thicker types ofplating. It is understood that other metals and other applicationprocesses may be used to plate barrel 40 and plunger bore 48.

The metal plating of first layer 56 may be impregnated with one or moretribological materials (Step 406). Tribological materials, such as PTFEor other materials, such as materials containing MoS₂, may beimpregnated in the metal plating of first layer 56 in order to increasethe lubricity of barrel coating 52. It is understood that othertribological materials may be used to impregnate the metal plating offirst layer 56, and also that the tribological materials may beimpregnated in the metal plating before being applied to the barrel 40.

Support layer 62 of plunger coating 54 may then be applied to plungersubstrate 60 of plunger 50 using a PVD process (Step 408). Support layer62 may include a tribological material such as W-DLC. Thetungsten-to-carbon ratio of W-DLC for use with support layer 62 may beset to any desirable ratio for achieving a desired hardness andtoughness for absorbing stresses from thermal expansion duringtemperature changes (e.g., when pump 10 is primed). It is understoodthat other tribological materials may be used, such as chromium carbide.In this example, the PVD process may include unbalanced magnetronsputtering in order to achieve a more dense coating. Further, PVDprocesses are advantageous because they can be used to applytribological materials, such as DLC coatings, to metal substrates, suchas stainless steel, at low temperatures. This low-temperature process isadvantageous for applying DLC coatings to stainless steel substratesbecause the risk of heating the substrate to a temperature that canalter the substrate's tempering is reduced. It is understood, however,that other PVD processes may be used to apply support layer 62.

Main layer 64 of plunger coating 54 may then be applied to plunger 50 ontop of support layer 62 (Step 410). Main layer 64 may contain atribological material such as ADLC. ADLC may be applied using aplasma-assisted CVD process. Plasma-assisted CVD may be performed atlower temperatures than other CVD processes, thereby reducing the riskthat the tempering of plunger substrate 60 (e.g., containing stainlesssteel) is altered. It is understood that other application processes maybe used to apply main layer 64.

Break-in layer 66 of plunger coating 54 may then be applied to plunger50 on top of main layer 64 (Step 412). Break-in layer 66 may be asacrificial layer containing a tribological material, such as W-DLC, andmay be configured to wear away from main layer 64 and be deposited ontoplunger bore 48. A PVD process may be used to apply break-in layer 66.The acetylene partial pressure of the PVD process may be increased andthe tungsten-to-carbon ratio of W-DLC may be decreased in order tosoften break-in layer 66 and to allow it to transfer to plunger bore 48.It is understood that other tribological materials and applicationprocesses may be used to apply break-in layer 66. Although break-inlayer 66 may be applied to plunger bore 48, break-in layer 66 may beapplied instead to main layer 64 in some embodiments because PVDprocesses may involve a line-of-sight application of tribologicalmaterials to a work surface. Accordingly, in some embodiments, theapplication equipment for performing PVD processes may more effectivelyapply a tribological coating to an outer surface of plunger 50 asopposed to an inner surface of barrel 40, such as plunger bore 48.

Break-in layer 66 may include other tribological materials such asTiMoS₂ and/or ceramic materials, which may be applied using otherapplication processes, alternatively or in addition to a DLC material.For example, in other embodiments, step 412 may include applying aceramic material using a thermal spray process instead of applying amaterial using a PVD process. In particular, a zirconia thermal spraymay be used. The zirconia thermal spray be lubricious at cryogenictemperatures, but may be thicker than materials applied using the PVDprocess. If a ceramic material is applied at step 412, such as azirconia thermal spray, additional steps may be required in order togrind, polish, or otherwise finish the surface of the ceramic material.

After coating both barrel 40 and plunger 50, plunger 50 may be insertedinto plunger bore 48 of barrel 40, and barrel 40 and plunger 50 may beassembled into barrel assembly 34 and into pumping mechanism 30. Plungerbore 48 may then be fluidly connected to a source of cryogenic fluid,such as liquefied natural gas, helium, hydrogen, nitrogen, or oxygen,and pump 10 may be driven to slide plunger 50 through plunger bore 48 totransfer break-in layer 66 from main layer 64 to plunger bore 48,thereby breaking-in barrel 40 and plunger 50 (Step 414).

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the pump of the presentdisclosure. Other embodiments of the pump will be apparent to thoseskilled in the art front consideration of the specification and practiceof the pump disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope beingindicated by the following claims and their equivalents.

What is claimed is:
 1. A pump comprising: at least one pumping mechanismincluding: a barrel formed of a barrel substrate having a bore; and aplunger formed of a plunger substrate and slidably disposed within thebore in the barrel; and a coating disposed on the plunger, the coatingincluding a support layer disposed on the plunger substrate, the supportlayer containing a support layer tribological material containing atleast one of a DLC material and chromium carbide, a main layer disposedon the support layer, the main layer containing a main layertribological material, and a sacrificial break-in layer disposed on themain layer, the break-in layer containing a break-in layer tribologicalmaterial.
 2. The pump of claim 1, wherein the main layer tribologicalmaterial contains amorphous diamond-like carbon (ADLC).
 3. The pump ofclaim 1, wherein the break-in layer tribological material contains atleast one of a diamond-like carbon (DLC) material, titanium molybdenumdisulfide (TiMoS₂), or a ceramic material.
 4. The pump of claim 1,wherein the bore is coated with a metal plating.
 5. The pump of claim 4,wherein the metal plating includes nickel or chrome.
 6. The pump ofclaim 4, wherein the metal plating is impregnated with a tribologicalmaterial.
 7. The pump of claim 6, wherein the metal plating tribologicalmaterial includes polytetrafluroethylene (PTFE).
 8. The pump of claim 1,wherein the barrel substrate includes stainless steel and the plungersubstrate includes stainless steel or a ceramic.
 9. The pump of claim 8,wherein stainless steel includes 17-4 PH H1150-M stainless steel.
 10. Amethod of forming a pump, the method comprising: providing a plunger;applying a coating to the plunger, wherein applying the coating includesapplying a support layer of the coating to a substrate of the plunger,the support layer including a support layer tribological materialcontaining at least one of a DLC material and chromium carbide, applyinga main layer of the coating to the support layer, the main layercontaining a main layer tribological material; and applying asacrificial break-in layer of the coating to the main layer, thebreak-in layer containing a break-in layer tribological material; andslidably disposing the plunger within a bore, the bore being located ina barrel.
 11. The method of claim 10, wherein the main layertribological material contains amorphous diamond-like carbon (ADLC). 12.The method of claim 10, wherein the break-in layer tribological materialcontains at least one of a diamond-like carbon (DLC) material, titaniummolybdenum disulfide (TiMoS₂), or a ceramic material.
 13. The method ofclaim 10, further including applying a metal plating to the bore. 14.The method of claim 13, wherein the metal plating is impregnated with atribological material.
 15. The method of claim 10, further includingwearing away a portion of the break-in layer from the main layer therebytransferring material to the bore in the barrel.
 16. A pump comprising:at least one pumping mechanism configured to be fluidly connected to asource of cryogenic fluid, the at least one pumping mechanism including:a barrel formed of stainless steel, the barrel including a bore, thebore having a nickel-plated surface; and a plunger formed of stainlesssteel, the plunger being slidably disposed within the bore in thebarrel, wherein the at least one pumping mechanism is configured topressurize the cryogenic fluid between the plunger and the barrel; and acoating disposed on the plunger, the coating including: a support layercontaining a diamond-like carbon (DLC) material; a main layer disposedon the support layer, the main layer containing amorphous diamond-likecarbon (ADLC); and a sacrificial break-in layer disposed on the mainlayer, the break-in layer containing a DLC material.